Amines have long been known to be extremely and objectionably odorous even in very low concentrations in air. Their odors are detectable in air in concentrations as low as a few parts per billion by volume, or even in fractions of a part per billion. Furthermore, amines are produced by many natural as well as industrial processes or operations. For example, the bacterial decomposition of nitrogenous organic matter produces amines, and therefore amines are present in the atmospheric effluents from sewage and garbage, especially when these materials include wastes from meat and fish. Monomethyl amine occurs in herring brine, in the urine of dogs after they eat meat, and in certain plants. Trimethyl amine is present in menstrual blood and in urine. In addition, amines are widely used in chemical synthesis, and as such inevitably escape to the atmosphere as a result of small leaks, spillages, or displacement by other gases or liquids during the cleaning of equipment in which amines had been processed, transferred, or stored. Trimethyl amine is also formed during the distillation of sugar beet residues. Monomethyl amine and trimethyl amine have pungent, fishy, ammoniacal odors.
The amines that are gaseous at ordinary temperatures and pressures include monomethyl amine (normal boiling point -6.3.degree. C.), dimethyl amine (boiling point 7.degree. C.) and trimethyl amine (boiling point 3.5.degree. C.) Amines have very low odor threshold concentrations. For example, G. Leonardos, J. Air Pollution Control Association, Vol. 19, p. 91 (1969) has reported the odor threshold of monomethyl amine to be 0.021 parts per million by volume, and the odor threshold of trimethyl amine to be 0.00021 parts per million by volume. Most amines that are liquids or solids at 15.degree. to 25.degree. C. and normal atmospheric pressure have sufficiently high vapor pressures and sufficiently low odor thresholds so that their vaporization generates strong and unpleasant odors. For example, putrescine (tetramethylene diamine) is a colorless oil that boils at about 160.degree. C. at normal atmospheric pressure. It occurs in decaying animal tissues and vaporizes sufficiently to generate the typical odor of protein putrefaction from which it derives its name. Similarly, cadaverine (pentamethylene diamine) is a colorless syrupy liquid with a normal boiling point of about 180.degree. C. It is produced by bacterial action on meat and fish and it also occurs in cholera discharge. It, too, vaporizes sufficiently to generate the type of putrid odor from which it derives its name.
Activated carbon is the most effective wide-spectrum medium for removal of odorous matter from air or other gas streams (A. Turk, "Adsorption," Chapter 8 in AIR POLLUTION, Third Ed., Vol. IV, A. Stern, ed., Academic Press, New York, 1977). The deodorizing action of carbon proceeds by means of physical adsorption. Since carbon is a non-polar substance, it does not exhibit preference for polar materials, such as water, but instead is selective in adsorption largely on the basis of the boiling points of the components of a mixture of gases and vapors. Materials of higher boiling points are adsorbed in preference to those of lower boiling points. As expressed by J. W. Hassler in his book, PURIFICATION WITH ACTIVATED CARBON, Chemical Publ. Co., New York, 1974, page 12, "vapors that readily condense to a liquid state will be more readily adsorbed than gases that liquefy only if and when cooled to a low temperature." When odorous materials are present in air at ordinary temperatures, activated carbon is an economically effective medium for odorants whose normal boiling points are above about 60.degree. C. Thus, activated carbon is not an economically practical medium for the removal of ammonia (boiling point -33.degree. C.), or the low-boiling amines cited above, but it is an economically practical medium for the removal of putrescine or cadaverine, which have high boiling points. It is assumed in all these comparisons that the activated carbon is not impregnated with any chemical reagent, and therefore operates by physical adsorption.
The capacity of activated carbon for the removal of amines of low boiling point can be enhanced by impregnating the carbon with a chemical reagent. Amines are weak bases, which can be rendered odorless by reaction with strong nonvolatile acids. Thus, if the carbon is impregnated with phosphoric acid, its capacity for the removal of amines of low boiling point is greatly enhanced. The salt formed, an alkylammonium hydrogen phosphate, remains on the carbon and the treated air has thus been deodorized. Another effective impregnant for carbon is zinc acetate. The zinc ion forms complexes with amines, which likewise remain on the carbon, thus contributing to the deodorization of the treated air. However, the phosphoric acid, zinc acetate, or any other impregnant on the carbon occupies some of surface area and pore volume of the adsorbent, and thus reduces its capacity for physical adsorption. For amines of higher boiling points, such as putrescine and cadaverine, the enhanced capacity offered by an impregnant such as phosphoric acid or zinc acetate is offset by the reduced capacity for physical adsorption, and the result may be no net gain or even a net loss. Furthermore, many air streams that contain amines are also contaminated with other malodorous gases and vapors, such as mercaptans, sulfides, and fatty acids. These vapors are not trapped at all by the impregnants that are effective for amines, so that such impregnations can only reduce the capacity of the carbon for these other malodorants.